This this this.

Get some very thin nails. Take your best guess at where a stud is by hitting the wall with your hand or use an outlet as described in multiple other posts. Hit it hard if you need to. Tap a brad nail in. If you feel resistance once it penetrates the drywall or plaster, you hit a stud. If not, pull the nail and try again just a bit further left or right. If you are hanging a shelf cover the unnecessary holes with the shelf.